Beam‐beam effects in HERA were studied with a self‐consistent beam‐beam simulation by using the particle‐in‐cell method. A remarkable agreement between the experimental measurement and the simulation result was observed on the emittance growth, luminosity reduction, and coherent beam‐beam tune shifts. The simulation study also showed that the chaotic coherent beam‐beam instability could occur in HERA and this collective beam‐beam instability could be avoided with a slightly different tune. © 2003 American Institute of Physics